Microwave coupling arrangements



June 5, 1956 H. F. ENGELMANN ET AL 2,749,521

MICROWAVE COUPLING ARRANGEMENTS Filed March 5, 1952 INVENTORS HERBERT F.ENGELMANN JOHN A. KOSTRIZA United States Patent MICROWAVE COUPLINGARRANGEMENTS Herbert F. Engelmann, Mountain Lakes, N. J., and John A.Kostriza, New Dorp, N. Y., assignors to International Telephone andTelegraph Corporation, a corporation ofMaryland Application March 5,1952, Serial No. 274,855

1 Claim. (Cl. 333-11 This invention relates to microwave transmissionsystems and more particularly to coupling arrangements equivalent to themagic T type of coupler.

In the patents to H. F. Engelmann, No. 2,654,842, dated October 6, 1953,and to D. D. Grieg and H. F. Engelmann, No. 2,721,312, dated October 18,1955, a new principle of microwave transmission is disclosed, comprisingin its simplest form twoconductors, the first as a line conductor andthe second as a ground conductor, spaced close together in substantiallyparallel. relation. The so-called ground conductor, which may be atground potential or some other given potential, is preferablyconsiderably Wider than the line conductor so that the surface thereofprovides in effect a mirror image reflection of the line conductor,whereby the distribution of the electric and magnetic field between the.conductors is substantially the same as the distribution between oneconductor. and the neutral plane of a theoretically perfeettwo-conductor parallel system. Small variations in size and shape ofthe. line conductor may produce variations in the characteristicimpedance of the system but the field distribution with respect to theground conductor is not materially disturbed. Likewise, certainvariations in the surface of the ground conductordo not: materiallydisturb the field distribution with respect to the surface thereof sincesuch variations either neutralize each other or do not adversely aliectthe field distribution between the two conductors. By this system,microwaves can be propagated in substantially the TEM mode along theline-ground conductor system since the microwaves flow along the opposedsurface or skin portions of the conductors which define the opposedconductive boundaries for the concentrated regions of theelectro-magnetic field.

One of the objects of this invention is to provide an equivalent magic Tcoupling arrangement employing either the line-ground conductor featureof the transmission system referred to above or printed parallelconductors.

One of the features of the invention is the simplicity of constructionand the economy of cost of the coupler as compared to the waveguidemagic T coupler. The coupler according to the present inventioncomprises a dielectric sheet containing on opposite sides thereof theconductor configurations desired. For example, the conductorconfiguration on one side may comprise a closed conductor loop andbranch conductors disposed laterally thereof at selected points on theloop. The conductor configuration on the other side may be identical tothat on the one side or it may comprise a layer of conductive materialso as to present a planar conductive surface parallel to the plane ofthe conductive loop. If desired, the loop and branch conductors may beso supported relative the planar conductor as to have air dielectrictherebetween.

The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, wherein: a

Fig. 1 is a view in plan of a loop form of coupling between.

arrangement in accordance with the principles of this invention;

Fig. 2 shows a cross-sectional view with parts broken away takensubstantially along line 22 of Fig. 1;

Fig. 3 shows across-sectional view similar to Fig. 2 of a parallel striploop coupler; and v Fig. 4 is a view in plan of a variation of the loopcoupler shown in Figs. 1 and 2.

Referring to Figs. 1 and 20f the drawings, a loop form of coupler isshown comprising transmission lines of the line-above-ground type.Preferably the microwave transmission line is of the printed circuittype comprising a first or line conductor land a second or groundconductor 2 with a layer 3 of insulating material there- The. conductivematerial may bev applied to the layer of insulation, such. aspolystyrene, polyethylene, Teflon, or other flexible insulation of highdielectric quality, in the form of conductive paint or ink, or theconductivematerial may be chemically deposited, sprayed through astencil or dusted. onto selected prepared surfaces of the installationaccording to known printed circuit techniques. For relatively shortlengths of lines or loop configurations, the conductive strips may becut and applied by a clie-stan1ping operation. In some cablemanufacturing-processes, the insulation may be extrudedandsimultaneously or later coatedon the two sides thereof 'withconductive material of the desired thickness and widths. Where thewidths. of the two coatings are the same and it is desired to reduce thewidth of one of the coatings, the portions of the two coatings that areto beretained may be coated with a chemically inert material exposingthe parts to be removed, and thereafter passed through an etching bath,whereupon the exposed portionsof the conductivecoatings are removed.While the two conductive coatings 1 and 2- are shown in crosssection tobe substantially rectangular, they may comprise different shapes so longas one of the conductors presents a planar surface with. respect to theother. Preferably, the ground conductor is two to three times the widthof the line conductor 1, although two such conductors of equal widthwhen printed on a sheet of dielectric will function as a two-wireparallel system. The electric field concentrated for a ratio of widthsof line to ground conductors of l to 3 is found to range from about percent to approximately 99 per cent. From the foregoing it is clear that anarrow ground conductor may be used without much radiation loss, andwhere it can be three or more times the width of the line conductor, anexceptionally low loss transmission line is assured. For furtherinformation on various line-above-grount cable constructions, referencemay be had to the aforementioned Patent No. 2,721,312.

From the foregoing description of a line-above-ground transmissionsystem, it will be clear that such a conductor arrangement is similar toa rectangular waveguide especially where the line conductor is of flatstrip form. Whether or not the line conductor is round, oval, or fiat,wave propagation is predominantly of one polarization, that is, theconcentration of lines of electrical force 4 are substantiallyperpendicular to the opposed surfaces of the line and ground conductors.It will be understood from this that the spacing of the two conductorscornprises a very small fraction of a wavelength of the micro wavepropagated therealong and that this spacing is usually less than thediameter or width of the line conductor 1, thus incurring a highconcentration of the electric field directly beneath the line conductor.

The loop coupler shown in Figs. 1 and 2 comprises a loop 5 of the lineconductor 1 connected with four radially disposed branches 6, 7, 8, and9. The branches 6 and 7 are connected at diametrically opposite pointson the loop While the branches 8 and 9 are connected at spaced points atone side of the loop. The mid circumference of the loop is selected tobe one and a half wavelengths so that the distance between branches 6and 7 will be three-quarters of a wavelength. The location of branches 8and 9 is such that these branches are spaced a quarter of a wavelengthfrom adjacent branches. The loop and the branches 6, 7, 8, and 9 overliethe planar surface of conductor 2, the sheet of dielectric material 3being disposed therebetween as the supporting medium. By disposing theline conductor of the loop and branches a small fraction of a quarterwavelength above the planar surface of conductor 2, the electric field 4is highly concentrated between the opposed surfaces of the lineconductors and the planar surface of the line conductor 2. This fieldapproximates the field of the TEM mode so that microwaves are propagatedalong the skins of the conductors which bound the electric field. Anyphysical discontinuities in the line conductors 1 may tend to set uphigher order modes but due to the close spacing of the conductors 1 withrespect to the planar surface of conductor 2 these higher order modesare quickly attenuated.

Assuming that the source of microwave energy is received over branch 6,there is an outward flow of microwave energy over adjacent branches 7and 8. Likewise, if a source of microwave energy is coupled to branch 7an outward flow will be obtained over adjacent branches 6 and 9. v

In Fig. 3 a cross-section of a parallel strip line arrangement is shownsimilar to the cross-section of Fig. 2, it being understood that thestrip 2a corresponds to the planar conductor 2 in the embodiment ofFigs. 1 and 2. As shown in the cross-section the line conductor 1a andthe so-called ground conductor 2a are of substantially the same width incross-section and are held in true parallelism by a sheet of dielectricmaterial, such as indicated at 30. The field distribution between theparallel strips is indicated at 4a. This field is highly concentrated inview of the fact that these conductor strips are purposely disposed avery small fraction of a quarter wavelength. The operation of theparallel strip loop coupler follows the same as for the line-planarconductor system of Figs. 1 and 2.

In Fig. 4, the loop is shown to be in the form of a rectangle 10. Thisform is shown to indicate that the loop need not be circular but maycomprise any configuration so long as the current flow follows a closedcircuit having a circumference dimension equal to one and a halfwavelengths. The loop circumference, however, may be of other lengths solong as the lengths provide for max and null conditions at the junctionsin accordance with the illustration of Fig. 1. For example, the loopdimensions may be five quarters wavelength for one half of the loop,with three quarters wavelength between branches 6, 8; 8, 9; and 9, 7.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made by way of example only and not as a limitationto the scope of our invention as set forth in the objects thereof and inthe accompanying claim.

We claim:

A microwave coupler comprising first and second conductors only, aplanar dielectric sheet supporting said first conductor on one surfacethereof and said second conductor on the opposite surface thereof, thespacing between said conductors being a minor fraction of a wavelengthat the mean frequency of waves applied to said coupler, said firstconductor being a flat strip conductor in the form of a loop with aplurality of branches extending from said loop, two branches atdiametrically opposite points, and two other branches each at a pointdistant in the order of one-quarter of a wavelength from the nextadjacent branch, the loop having a mid-circumference in the order of oneand a half wavelengths, the loop and its branches lying substantially ina given plane on said dielectric sheet, said second conductor being aplanar sheet underlying the entire loop and at least a portion of eachof said branches adjacent said loop.

References Cited in the file of this patent UNITED STATES PATENTS2,409,449 Sanders et a1. Oct. 15, 946 2,445,895 Tyrrell July 27, 19482,611,822 Bliss Sept. 23, 1952

